1. Field of the Invention
The present invention relates to a cooling water passage structure in a cylinder head of an internal combustion engine.
2. Description of the Related Art
A multi-cylinder engine is generally constructed such that a plurality of intake ports and a plurality of exhaust ports are formed inside a cylinder head, and an intake manifold for distributing intake air and an exhaust manifold for collecting exhaust gases are connected respectively to an intake-side lateral surface and an exhaust-side lateral surface of the cylinder head. In another known construction, an exhaust collecting portion for collecting exhaust gases is also formed inside the cylinder head, and a single exhaust pipe is connected to the exhaust-side lateral surface of the cylinder head. The multi-cylinder engine provided with the exhaust collecting portion formed inside the cylinder head has the following advantages. Because of no need of separately providing the exhaust manifold, the size of the entire engine can be reduced. Further, because heat released from the exhaust gases can be suppressed, the catalyst temperature can be more quickly raised for earlier activation in warming-up. However, the exhaust gases need to be properly cooled in order to prevent thermal deterioration of a catalyst, which may be caused due to an excessive temperature rise. Moreover, a countermeasure is required to protect, against thermal damage, bolts for fastening the exhaust pipe that is coupled to the cylinder head immediately downstream of the exhaust collecting portion.
Additionally, in the cylinder head provided with the exhaust collecting portion formed therein, when a large cooling water passage is formed around the exhaust collecting portion, a boss portion for the bolt for fastening the exhaust pipe, which is disposed immediately downstream of the exhaust collecting portion, is projected into the cooling water passage. Within the cooling water passage, therefore, vortexes are generated downstream of the boss portion for the bolt, thus causing cooling water to stagnate. Hence, the cooling effect with the cooling water passage is reduced. To overcome the above-mentioned problem, an invention is proposed in which an auxiliary cooling water passage is formed such that the cooling water in a cylinder block is caused to flow into the cooling water passage and to direct toward a vortex generation region (see Japanese Unexamined Patent Application Publication No. 2009-115031).